Whenever a signal travelling along a transmission line encounters some change, i.e., discontinuity, in the line characteristic, a portion of the signal is reflected back towards the sending end of the line. The nature of the reflection signal is determined by the discontinuity characteristic, which might be anywhere between a short circuit and an open circuit.
Time domain reflectometers provide a means to determine the characteristics of faulty and normal electrical transmission lines, by sending an excitation signal, receiving a reflected response and analysing the response.
FIG. 1 (Prior Art) shows schematically the external appearance of a portable time domain reflectometer, TDR. Other forms are possible, for instance a fixed installation wherein the TDR is automatically controlled and does not require any user keypad or visual display.
FIG. 2 (Prior Art) shows the typical blocks that make up a TDR. These are:
Power Supply—which provides the necessary power to the various circuits;
Processor/Memory—which, as in many examples of modern instrumentation, provides overall operational control, processing of user actions, control of information provided to the user, management for the generation of test signals, management for the acquisition of measured signals, mathematical analysis of measurements and the application of signal processing algorithms.
In this context, the term “user” might also mean a separate piece of linked system control equipment as well as a human operator;
User Interface(s)—in a portable TDR these would typically be a keypad for entering commands/data and a screen for the display of measured signal responses, derived measurements and system information. In a fixed installation, the user interface might consist of a serial communication port such as a RS232 or USB port;
Test Signal Generator—provides a test signal for application to the transmission line (cable) under test. In practice it might be in the form of a voltage source or a current source. It might also be presented in either a single-ended (unbalanced) or balanced form.
Line Feed Resistor(s)—this or these provide the correct matching impedance for the line being tested. As previously mentioned, when a signal travelling along a transmission line encounters a change in characteristic, a reflection occurs. This is also true for a reflected wave returning to the TDR instrument. The instrument should therefore present an impedance characteristic sensibly close to the impedance characteristic of the line under test, if it is to avoid causing further unwanted signal reflections. The line feed resistor(s) is or are therefore provided to give the correct matching characteristic for the line under test. Multiple selections may be provided to cater for various line types.
In practical realisations, the test signal generator may consist of a voltage generator which is then arranged in series with the line feed resistor(s) or a current generator arranged in parallel with the line feed resistor(s). These are equivalent as per the well known Thevenin and Norton equivalent forms. Also the signals might be provided, and the responses measured, as either a single-ended or a balanced form which are well known in measurement systems. The later analysis is presented in the single-ended (unbalanced) form although this is easily extended to the balanced form, as is well known.
Additionally, some form of dc isolation might be provided between the TDR instrument circuitry and the connectors providing the access to the line (cable) under test. Typically this is done by the use of capacitors whose value is chosen to have minimal effect on the signals generated to and received from the line (cable) under test. If these capacitors do have a significant effect, it can be compensated for by the use of traditional analogue or digital filter techniques.
Signal Measurements—This block provides the ability to capture the electrical signals appearing on the TDR line (cable) under test, access point. It can therefore acquire signals with or without the cable actually connected. Typically an input amplifier of suitable impedance when considered in conjunction with the line feed resistor(s) will pass the signal to an analogue to digital converter (ADC) which is used to capture signal values on a point by point basis in time, which are then passed via the processor to a memory store for later evaluation.
In a practical TDR, the effect on the measured signal due to any dc isolation may again be compensated for by use of traditional analogue or digital filter techniques. Also, the input amplifier/ADC circuit may be presented in either single-ended (unbalanced) or balanced configurations.
Access Point—provides the terminal connections such that the cable under test can be connected to the instrument's test and measurement circuitry.
Traditional TDRs use a substantially rectangular pulse, or pulses that are smoother in nature such as a half-sine shape or a raised cosine shape. Other TDRs use a step waveform, which does not return to zero over the duration of the measurement.
One characteristic of an electrical transmission line is the rise/fall charge transients that can be seen when signals are applied to its input. This effect can hinder the observation and analysis of weak reflection signals in TDR responses. It is thus of fundamental interest to substantially remove this unwanted characteristic.
A variety of hardware and software techniques exist to reduce the unwanted effect. These techniques may be referred to as compensation, correction, balance or signal processing, amongst others.
One technique is to make measurements (baseline reference results) on a known good line of the same type being tested and then to subtract this reference signal from the subsequent line test response. The resulting response then primarily consists of only the desired reflection response. This method has the disadvantage of requiring access to known good lines, which may not be convenient or even possible.
Another technique provides an adjustable hardware balance network, which attempts to provide an approximation of the unwanted effect, which can be subtracted from the overall response. In practice, the unwanted characteristic, which is described by a combination of Bessel functions and exponential decays, is not easily characterised by simple hardware circuits and the balance effectiveness is of only limited value. In any case it is desirable to replace hardware solutions with software solutions, where possible. This principle is illustrated in FIG. 3.
Other techniques based on signal processing software alone may be applied.